Version: 2020.3
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Dispatching Events
Synthesizing Events

Handling events

The event handling sequence

Events in UI(User Interface) Allows a user to interact with your application. More info
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Toolkit are similar to HTML events. When an event occurs, it’s sent along the propagation path in the visual element tree. The event isn’t only sent to the target visual element, but to all elements within the propagation path.

The event handling sequence is as follows:

  1. Execute event callbacks on elements from the root element down to the parent of the event target. This is the trickle-down phase of the dispatch process.
  2. Execute event callbacks on the event target. This is the target phase of the dispatch process.
  3. Call ExecuteDefaultActionAtTarget() on the event target.
  4. Execute event callbacks on elements from the event target parent up to the root. This is the bubble-up phase of the dispatch process.
  5. Call ExecuteDefaultAction() on the event target.

As an event moves along the propagation path, the Event.currentTarget property updates to the element currently handling the event. Within an event callback function:

  • Event.currentTarget is the visual element that the callback registers on.
  • is the visual element where the original event occurs.

For more information, see Dispatching events.

Registering an event callback

Registering an event callback lets you customize the behavior of an individual instance of an existing class, such as reacting to a mouse click on a text label.

Each element along the propagation path (except the target) can receive an event twice:

  • Once during the trickle-down phase.
  • Once during the bubble-up phase.

By default, a registered callback executes during the target phase and the bubble-up phase. This default behavior ensures that a parent element reacts after its child element. For example, if you want a parent element to react before its child, register your callback with the TrickleDown.TrickleDown option:

// Register a callback for the trickle-down phase
myElement.RegisterCallback<MouseDownEvent>(MyCallback, TrickleDown.TrickleDown);

This informs the dispatcher to execute the callback at the target phase and the trickle-down phase.

To add a custom behavior to a specific visual element, you must register an event callback on that element. For example, the following code registers a callback for the MouseDownEvent:

// Register a callback on a mouse down event

Your callback signature should look like this:

void MyCallback(MouseDownEvent evt) { /* ... */ }

You can register multiple callbacks for an event. You can only register the same callback function on the same event and propagation phase once. To remove a callback from a VisualElement, call the myElement.UnregisterCallback() method.

Adding custom data to the event callback

You can send custom data along with the callback to an event. To attach custom data, you must extend the call to register the callback.

The following code registers a callback for the MouseDownEvent and sends custom data to the callback function:

// Send user data along to the callback
myElement.RegisterCallback<MouseDownEvent, MyType>(MyCallbackWithData, myData);

The callback function should return this signature:

void MyCallbackWithData(MouseDownEvent evt, MyType data) { /* ... */ }

Responding to events with custom controls

If you’re implementing custom controls, you can respond to UI Toolkit events in two ways:

  • Registering an event callback.
  • Implementing a default action.

How you choose to respond to events depends on the situation.

The differences between callbacks and default actions are:

  • Callbacks must register on instances of the class. Default actions run as virtual functions on the class.
  • Callbacks execute for all visual elements in the propagation path. Default actions run only for the event target.
  • Callbacks might perform additional checks to determine whether they should react to an event. For example, a callback handling a mouse click might check if the element is the target of the event. Default actions can skip this step.
  • Default actions have a slight performance advantage because they don’t require a lookup in the callback registry during the propagation phase.

Implementing a default action

A default action applies to all instances of the class. A class that implements default actions can also have callbacks registered on its instances.

When a class implements a default action, it must derive a new subclass of VisualElement and implement either the ExecuteDefaultActionAtTarget() method, the ExecuteDefaultAction() method, or both.

Default actions execute on each instance of a visual element sub-class when that instance receives an event. To customize default actions, you can override ExecuteDefaultActionAtTarget() and ExecuteDefaultAction(), as shown in the example below:

override void ExecuteDefaultActionAtTarget(EventBase evt)
    // Call the base function.

    if (evt.GetEventTypeId() == MouseDownEvent.TypeId())
        // ...
    else if (evt.GetEventTypeId() == MouseUpEvent.TypeId())
        // ...
    // More event types

Implementing your default actions in ExecuteDefaultAction() allows you to stop or prevent the execution of a default action.

If you want the target default action to execute before its parent callback, implement the default actions in ExecuteDefaultActionAtTarget().

You should view default actions as the behaviors that an element type should have when it receives an event. For example, a checkbox should toggle its state in response to a click event. To execute this, you can override a default action virtual function, instead of registering callbacks on all checkboxes.

Best practices for custom controls

Implementing behaviours

You should implement behaviors from your element with default actions. You can call PreventDefault() in a callback attached to an instance to cancel default element behaviors.

Additional benefits of implementing behaviors as default actions are:

  • Default actions don’t require a lookup in the callback registry.
  • Instances without callbacks don’t enter the propagation process.

For greater flexibility, you can execute default actions of the event target at two moments during the event dispatch process:

  1. Between the trickle-down and the bubble-up propagation phase, immediately after execution of the target callbacks, override ExecuteDefaultActionsAtTarget().
  2. At the end of the event dispatch process, override ExecuteDefaultActions().

Default actions on a class

Implement your class default actions in ExecuteDefaultActions(), if possible. This allows more options to override the class. You can call PreventDefault() to override the class during the trickle-down phase or the bubble-up phase of the event propagation process.

You must stop propagation of an event during a default action if the event shouldn’t propagate to the parent element. For example, a text field receives a KeyDownEvent that modifies its value, such as the Delete key to delete content. This event must not propagate to the parent visual element. Use ExecuteDefaultActionsAtTarget() to implement a default action, and call StopPropagation() to make sure the event isn’t processed during the bubble-up phase.

Default actions only execute for an event target. For a class to react to events that target their child or parent elements, you must register a callback to receive the event either during the trickle-down or the bubble-up propagation phase. Avoid registering callbacks in your class to improve performance.

Stopping event propagation and cancelling default actions

When handling an event inside a callback or a default action, you can stop further event propagation and the execution of default actions. For example, a parent panel could stop propagation during the trickle-down phase to prevent its children from receiving events.

You can’t prevent the execution of the EventBase.PreDispatch() and EventBase.PostDispatch() methods inside the event class itself.

The following methods affect event propagation and default actions:


  • Stops the event propagation process immediately, so no other callbacks execute for the event. However, the ExecuteDefaultActionAtTarget() and ExecuteDefaultAction() default actions still execute.


  • Stops the event propagation process after the last callback on the current element. This ensures that all callbacks execute on the current element, but no further elements react to the event. The ExecuteDefaultActionAtTarget() and ExecuteDefaultAction() default actions still execute.


  • Prevents the event propagation process from calling the ExecuteDefaultActionAtTarget() and ExecuteDefaultAction() default actions. PreventDefaultAction() doesn’t prevent the execution of other callbacks, and is ineffective on ExecuteDefaultActionAtTarget() when called during the bubble-up phase.

  • 2018–11–02 Page amended
Dispatching Events
Synthesizing Events
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